Potassium channels are universally found in biological systems and play major roles in the cellular signal transduction process. The generally decrease the membrane excitability by clamping the membrane voltage near the potassium equilibrium voltage. Functional properties of the potassium channels can ultimately influence how the organism responds to its environment. Several different potassium channels have been characterized. Inward-rectifier potassium channels open in response to hyperpolarization and they are involved in maintaining normal resting potential. By influencing the resting potential, these inward-rectifier channels regulate the cellular excitability. Fore example, in heart muscle, the inward-rectifier channels greatly influence the rhythmicity, effectiveness of various antiarrhythmic agents, and the action potential conduction. The proposed research focuses on the cloned inward-rectifier potassium channel (KAT1) expressed in a heterologous expression system. The goal of the research is to understand how this channel opens and closes at the molecular level by developing a kinetic model to describe the behavior. The channel protein structure will be rationally mutated and the alterations in the function will be quantitatively assayed using electrophysiological methods. the preliminary results indicate that the opening of the KAT1 channel requires both hyperpolarization and some intracellular factors. The proposed research will identify the intracellular factors and investigate which amino acid residues of the channel involved in mediating the effects of these intracellular factors. The research proposed here will also investigate how the KAT1 channel opens in response to hyperpolarization. This research will examine if the structural and functional properties of the activation of the KAT1 channel favored by hyperpolarization are similar to those of the depolarization-activated potassium channels. By examining the molecular and biophysical properties to the KAT1 channel activated by hyperpolarization and by comparing the results with those obtained from depolarization-activated potassium channels, it should be possible to gain insights into the universal molecular principles involved in the voltage-dependent channel behavior. Furthermore, since the KAT1 channel is activated by the intracellular agonists and hyperpolarization, the results obtained will also be very relevant to the studies on various agonist-activated ion channels.